An estimated 300,00 patients will undergo coronary artery bypass graft (CABG) surgery in 1994. An increasing number of these patients are elderly, and we have just found that this patient population is particularly susceptible to early cognitive dysfunction after cardiac surgery. Neuropsychologic dysfunction in the immediate postoperative period is a disturbing clinical problem which often (in up to as many as 83% of patients) complicates successful surgery. We will test the hypothesis that hypothermia protects the brain from injury (as manifested in altered cognitive function) during cardiac surgery. This is the first prospective study of the protective effect of hypothermia on the postoperative incidence of neuropsychologic dysfunction in coronary artery bypass surgical (CABG) patients. 400 CABG patients will be enrolled over four years. The hypothesis will be tested by randomization of age stratified patients to normothermic (36%C) and moderate (28$C) hypothermic perfusion during CABG surgery. Hypothermia produces cerebral metabolic suppression and the degree to which the two temperature perfusions alter cerebral oxygen metabolism will be determined using Xe133 clearance and arterial - jugular bulb venous oxygen gradients to calculate the cerebral metabolic rate of oxygen (CMRO2). The primary aim of the investigation is to determine whether hypothermia protects the patient from cognitive dysfunction. The changes in CMRO2 as well as the time of hypothermic protection will be correlated with the changes in a serially administered battery of neurologic and psychometric tests. Change in cognitive function will be correlated with the reduction in CMRO2 and time integral (hypothermia) during CPB. A secondary aim of the investigation is to determine whether cognitive dysfunction leads to impairment in the quality of life and adds to hospital costs. This will be done by using a battery of tests developed by our investigators to measure quality of life and by a detailed cost analysis program also developed here to examine health care costs after cardiac surgery. New knowledge generated in this investigation will have immediate applicability to clinical cardiac surgery. The results may demonstrate the importance of maintaining cognitive function in terms of quality of life and economic measures. Furthermore, the information will expand our understanding of the protective effects of hypothermia against ischemic insult.